Trivalent chromium plating solution



United States Patent TRIVALENT CHROMIUM PLATING SOLUTION Jean Jacques Georges Icxi, Paris, France No Drawing. Application July 27, 1953, Serial No. 370,640

Claims priority, application France March 20, 1948 13 Claims. (Cl. 204-51) This invention is concerned with an electroplating solution of chromium, which allows obtaining very quickly a chromium coating of very good physical and mechanical properties.

The present application is a continuation-in-part of my co-pending application Serial Number 82,264 filed March 18, 1949, now abandoned.

More than a hundred patents have described electroplating solutions in which chromium is present in the form of chromic acid or salts of this acid, that it to say in the hexavalent state. These solutions give rise to a number of difliculties; many attempts have been made to overcome them by adding to these solutions small amounts of various organic and mineral compounds. But, whatever be the nature of such additions, their low proportions do not change the nature of these solutions which remain solutions of hexavalent chromium, as is clear to those skilled in the art. All the solutions so constituted have the known inconveniences of hexavalent chromium solutions, among others, the necessity that the surface to be plated be perfectly clean, and the necessity to operate at or about room emperature and with low current densities, generally no higher than 12 to 15 amperes per square decimeter.

As it will appear from this specification, the solutions according to the invention do not exhibit the above noted drawbacks.

Plating solutions in which chromium is present in the hexavalent state are poisonous, are corrosive, and liberate toxic gaseous products during electrolysis. These poisonous characteristics do not exist in the solutions according to the present invention.

The electro-plating solution according to the present invention is characterized in that it contains only at least one salt formed by a complex anion constituted by trivalent chromium and at least one organic acid and by a cation which is an amine base.

The anion of such complex salt is thus constituted by an atom of trivalent chromium associated with six radicals of a monocarboxylic organic acid, or three radicals of a di-carboxylic organic acid, these acid radicals being identical or different. This anion is thus trivalent, as the chromium is itself trivalent, and is consequently neutralized by a cation having three radicals of an amine base, these radicals being identical or different. In general, the organic acid has less than six atoms of carbon and the amine has at most six atoms of carbon.

It is not necessary to employ a single organic acid or a single amine base, to constitute a solution according to the present invention. According to the present invention, it is possible to employ a chromium salt of one organic acid and an amine base salt of another organic acid. It is also possible to employ a mixture of different salts of difierent amine bases.

According to Werners classification, described in the book Introduction 21 la chimie des complexes by Georges Urbain et Snchal, edited by Herman et fils, Paris 1913,

2 the salts as employed according to the present invention are represented by the general formula [Cr (acid radicals)] (amine) where acid radicals designate the radicals of at least one organic acid, having each less than six atoms of carbon and the number of which radicals being such that the sum of their valences is equal to six, and where amine designates the ammonium radical or the radicals of at least one amine having at most six atoms of carbon, the number of said amine basic radicals being such that the sum of their valences is equal to three. Ammonium is considered as an amine radical.

As these salts are intended to be used in solution, the acids and amines which are suitable are those which produce a soluble complex salt, that is to say acids whose number of carbon atoms is lower than six and amines whose number of carbon atoms is lower than six.

The expression complex salt is employed in the present specification and claims to denote a salt of the kind indicated hereinbefore, in the molecule of which the organic acid radicals are identical, and the amine radicals are also identical. I make use of the terms mixed complex salt to designate a salt in the same molecule of which are included diiferent carboxylic organic acid radicals and/ or amine radicals.

According to the present invention, in general, using a trivalent anion of chromium, an amount of chromium hydroxide corresponding to a molecule of chromium is dissolved in an amount of molecules of at least one carboxylic organic acid, n being the number of carboxylic groups in the acid radical.

Many actual tests under operating conditions made on an industrial scale have shown that such electro-plating solution according to the present invention can be employed at temperatures as high as the boiling point of the plating solution and with current densities higher than 300 amperes per square decimeter.

In usual practice, the current density is about 200 amperes per square decimeter.

The process according to the present invention is satisfactory for plating small work pieces under very high current densities.

In these plating solutions, all according to the present invention, chromium is present only in a complex anion. During electrolysis, the object to be plated being the cathode, there is first obtained the well known phenomenon according to which the complex anion is transferred to the anode and the amine radical to the cathode, which is always surrounded by an alkaline medium; as the complex anion cannot remain in this form, it is split down into a chromium ion and an acid ion which remains near the anode, the chromium being transported towards and plated on the cathode. As the latter is always alkaline, there is no need that the object to be plated be perfectly clean or scoured.

As the chromium must be completely present only as a complex ion, the proportions of the components used to prepare the solutions must be about those theoretically t in accordance with the formula of the salt to be prepared.

' of an insoluble chromium precipitate following a com- 3 plete allialinization of'the solution. Should such a precipitate be formed, 'it Will 'b'e sufiieient 'fO a dd "a fllfilfil amount of the salt of the amine base, or of the acid.

A solution according to the present invention may be'tibtaihedby'dire'ctlydissolving of water a complex salt 'd'fch'rcrfilufn df the kind 'a'bo've specified, with 'the'additionof a salt of an amine base if necessary.

This solution according to the present-invention may also be obtained indirectly. According -to a first embodiment, I-,p'repare a solution of an organic salt of trivalent chromium to which I add a solution of an organic salt of the selected amine base. I can also dissolve the amine base in the total necessary amount of organic acid, and thenI dissolve in this solution'the proper amount of hydroxide or basic carbonate of chromium.

I can also start from a solution of a mineral salt o f chromium such as t he violet sulphate of chromium, to whichI add the proper amounts of an organic salt of the selected amine and of an organic acid, and I treat the resulting solution by an amount oi": hydrated baryta correspondingto the amount'of sulphuric ion of the sulphate. -Barium sulphate being insoluble, the solution is free from mineral compounds.

The alkalinity or acidity of the raw solution obtained according to one or the other of the above mentioned processes may be corrected by a convenient addition of organic acid or amine base so as to bring the pH of the solution to a pH value of about 7.

According to a particular aspect of the present invention, platings may be obtained which are constituted by chromium alloys. To this end, I add to the solution of a complex salt of trivalent chromium, a soluble salt of a metal such as iron, nickel, cobalt, etc., which is compatible therewith, as shown by a preliminary test; the eleetroplatings of the other metal and of chromium are efiected simultaneously.

Moreover, the "chromium coatings obtaine'd from a solution according to the present invention are more or less carburized, that is to say contain a part of the chromium in the form of chromium carbide. Ihis characteristic is particularly outstanding when'the solution contains a formic complex of trivalent chromium and/ or when theanode is constituted by a graphite piece.

In using solutions according to the. present invention in chromium plating, the usual apparatus and operating conditions are employed, and the concentration during electroiysis, are substantially'normal, but the allowable current densities and temperature of the electrolyte are much greater than permissible in the prior art.

The'solutions according to the present invention are particularly useful when they are-employed in the socalled spot plating method in which an absorbent mass connected to the positive pole of a source of electric current is soaked in the solution and is applied to the object to be plated which-is connected to' the vnega'tive pole of the source or current. 'This method can be applied with apparatusof the kind described in my co pending application Serial No. 321,327.

The following examples, which are purely illustrative and should'not be construed in limitative sense, will'illustrate how a solution according to the present invention can be obtained.

Example 1 For a final volume of 1000 cm. 700 grams of "the chromium oxalate known as ammonium Gregory 's'alt, having the formula [Cr(C2O4)3] (NI-14):;

are dissolved in water,"with 'the' aid of heat; 'The'ehroin'ium is present as trivalent chromium.

Gregory "salt is described in Z'eitschrift f? Anorganische Chemie, v.11, p. 209 (1896) andv. 21,-pb8 618 99), in Pascal, Trait' 'deChi'mi"Miiirale,' vol. l0, pages 1074-1075, Paris 1933, and in an article by Bussy, Liebig's 'Annalen"derchimiefvol. 29,'page 31211839).

The solution thus obtained, electrolyzed under 150 amperes per square decimeter with platinum anode, yields a chromium coating of a light blue color and which exhibits very goodphysical and mechanical properties.

Example 2 The two following.industrialsolutionsare mixed in the proportion of:

6 volumes of :a'solution-of chromium 'formate containing 200 grams per literof theanhydrous salt, the chromium being presentas trivalent chromium, :and 4 volumes of a solution of ammonium 'formate at 16 Baum, containing 610 grarns-per liter of this ammonium formate salt.

After digestion for an hour at C. the resulting solution is brought to neutrality with pH .of 7 ,iin:the cold. The salt soobtained has.the-followingzformula:

its solution yields white coatings, bothwith the tank'and the spot methods.

Example '3 In the hot'way, 100 grams-of chromic=acid=titrating more than of'pure acid are reduced-byadding 1370 grams of formic acid of 90% by weighhrpartly neutralizecl by 200 grams of .ammoniawater 90f :26%"'by weight of NH3. When the reduction is completegthe solution is brought to neutrality-'byiadding asufficient amount of ammonia water, and :its volume is sacljusted to avalue of 600cm. There is'so obtained the same complex salt as in Example 2.

This solution, electrolyzed under 3250 amperes per square decimeter with graphitehanode, yieldsrchromium coatings :of verygood properties withia-surprising rapidity.

- In this example, the chromium is present :duringwelectrolysis 'as trivalent chromium. The :chromic acid 'is:';1'e-,

duced by the formic acid :to g-yieldi chromium vformate, according to the reaction:

molecules of i organic acid, where n is .the zvalence ofgthe organic 'acid that is .the number of =carboxylic groups in the acid radicals.

Thesolutionis partly neutralized by 60grams of ammonia of 26% by Weigh't'of NHs. After'it hasdissolvcd, there are ad'dd35 grams or hydrazine of 50% by weight and ,apprdximatelyifrom' 60 to 70 grams, of ammonia of 26% to attain neutrality(about-pI-I 7);'and-'the-volume bf'the solution is 'broli'ghttdlOOO cm witlr water.

The"comp1ex'salt"so obtained has'thefollowing T01- mula:

Example 5 To 500 cm. of an industrial solution of green (trivalent) chromium sulphate at approximately 30 Baum, containing 300 grams of the anhydrous salt for one liter, there are added at 70 C.: 500 cm. of a solution of ammonium formate at 16 Baum, containing 610 grams per liter of this salt, and 120 grams of formic acid of 90% by weight.

This solution is then treated by 350 grams of hydrated baryta, heated, and the precipitate of barium sulphate thus formed is filtered away. The solution so obtained is brought to pH of about 7, and 100 cm. of methyl alcohol are added thereto.

This solution, which contains a complex salt having the following formula:

yields white platings, even in a tank, under strong density of current.

Example 6 An amount of moist chromium hydroxide corresponding to 100 grams of (trivalent) Cr2O3 is dissolved in the cold in 200 cm? of water by means of 200 grams of 90% by weight formic acid which has less than six atoms of carbon. The resulting solution is rapidly mixed with a solution of formate of methylamine, having a pH of about 7, which is obtained by the neutralization of 400 grams of an aqueous solution of 33% by weight methylamine by means of about 200 grams of 90% by Weight formic acid. The formic acid is an organic acid having less than six atoms of carbon, and the methylamine is an amine having at most six atoms of carbon.

The solution is maintained during 15 to 30 minutes at a temperature of 7080 C. and is then alkalinized (pl-1:75) by the addition of a small calculated amount of the solution of the amine. which contains a complex salt having the following formula:

[Cr (H-COz) 6] (CH3NH2) 3 is brought to 1000 cm. and yields very good platings by electrolysis at temperature higher than 40 C.

Example 7 An amount of moist chromium hydroxide corresponding to 100 grams of 01203 is dissolved in the cold in 200 cm. of water by means of 200 grams of 90% formic acid. The resulting solution is rapidly mixed with a solution of formate of dimethylamine (CH3)2NH, obtained by the neutralization of 850 grams of an aqueous solution of dimethylamine of 25% by weight by means of about 200 grams of 90% formic acid.

The solution thus obtained is then treated as indicated in Example 6. It contains a complex salt having the following formula:

Example 8 To 6 volumes of an industrial solution of (trivalent) chromium acetate at 20 Baum, containing 250 grams per liter of the anhydrous salt, there are added 7 volumes of a solution of ammonium acetate at 11 Baum containing 450 grams of this salt per liter.

The resulting solution is maintained during six hours at a temperature of 70 C., then cooled and brought to a pH of about 7 by a careful addition of ammonia water.

The complex salt so obtained has the following formula:

[Cr(CH3CO2) 61(NH4) 3 The resulting solution,

Example 5 500 grams of Gregoryssalt of potassium as identified in Example 1 above, having the following formula:

are dissolved at -90" C. in 500 cm. of water. When this salt is dissolved, there are added 300 grams of sulphate of hydroxylamine SO4(NH2OH)2 and the mixture is agitated for half an hour. Then the temperature is brought to 6080 C. and this temperature and the agitation is maintained for one hour. The separated potassium sulphate is filtered ofi hot. The chromium is present as trivalent chromium.

The solution is then concentrated and again submitted to crystallization, and the separation of the potassium sulphate is achieved through an addition of 10% by volume of ethyl alcohol. The solution so obtained is brought to 1000 cm. with water. There is obtained a solution of the oxalic chromium complex salt of hydroxylamine, having the following formula:

[Cr(C2O4) 31(NH2OH) 3 Example 10 In the embodiment described in Example 2, formate of hydroxylamine may be partly substituted for ammonium formate and there are obtained the following com pounds:

[Cr(HCO2) s] (NH4) 2 (NHzOH) [Cr (HCO2) 6]NH4(NH2OH) 2 according to the respective proportions of hydroxylamine and ammonium formate. These proportions can be easily determined from the above given formulae, as is Well known by those skilled in the art. The chromium is present as trivalent chromium.

All these salts comprising an hydroxylamine ion yield very bright chromium platings.

Example 11 750 grams of tertiary amylamine (dimethyl-ethyl carbylamine) having the formula:

CH; CH;CH:--NH2 are added to 600 cc. of water and are neutralized by means of 250 grams of formic acid. The solution is heated to 80 C. and mixed with 500 cc. of a solution of violet sulphate of chromium (trivalent) at about 30 Baum, containing 300 grams per liter of the anhydrous CH; [C r (H-o 0,)[0 113-0 HFCZNHZ] I I CH: :4

Example 12 A complex salt having the following formula:

[Cr(HCO2) 6] (CH2 =CHCHz-NH2) 3 is obtained if the 750 grams of dimethyl-ethyl-carbylamineof Example 11 is replaced by 300 grams of allylamine (Z- ropenylamine) having the formula a eas-e9 7 the treatment being otherwise the same, except that the volume .of the resulting solution is brought to 1500 cc. The rom um is Pre en a val n chr mi m- Emm l 13 In the process according to Example 11, use is made of 275 grams of technical diethyl-triamine, having the formula:

instead of 750 grams of dimethyl-ethyl-carbylamine. The treatment is the same in other respects except that the solution is brought to 1 250 cc. and thepI-I adjusted to a value of 7. The complex salt obtained has the formula:

The chromium .is present asv trivalent chromium.

Example 14 An amount of moist chromium hydroxide corresponding to 100 grams of C1'203 (trivalent) is dissolved in a hot solution of 270 grams of maleic acid, having the formula:

[Cr(COzCH=CI-ICz 3] (NH4) 3 Example 15 The same operations are madeas in Example 14 but the two amounts of 270 grams of maleic acid are replaced by two successive amounts of 325 grams of propionic acid: CH3--CHz-CQ2H. -The following complex salt is obtained:

Example 16 If two.- successiveamounts.of.300 grams ofglutaric acid: CO2H,CH2-,CO2H. aresubstituted for. the 270 grams of maleic acid of Example 14, the following complex saltisobtained:

The two following industrial solutions aremixed in. the proportions-oflfi volumes;lof solution of ,(triy alent) chromium acetate 0f 20 Baum containing 250 grams per liter of the anhydrous saltjand 4 volumes of solution of ammonium formate of 16 Baum containing 610 grams per liter of this salt. The mixture is maintained during one hour at:80 'C..,and is then brought to a pH of 7 by a careful addition of ammonia water. The solution yields very good White coatings. The complex salt obtained has the formula:

In this example there are present two diiferentorganic acids havingeach less-than sixatoms of carbon, the number of carboxylic groups present in-said acid being such that the sum is equalto six, and: there are present radicals of an aminehaving at rn ost six. atoms of carbon,

the number of basic radicals being such that their sum is equal to three.

In all of the foregoing examples, in no case does the organic radical employed have more than five carbon atoms, nor does the amine employed have more thansix carbon atoms.

The chromium plating solutions which have been here described can be used for special electrolyzing methods, such as those known as spot or plugging or pencilling galvanoplasty. These solutions can be electrolyzcd using high current densities such as 200 to 300 amperes per square decimeter, and at temperatures as high as the boiling temperature of the solution itself. This is an important advantage of the plating solution here described.

Tests have been made for the purposes here in view of many mineral compositions of chromium. Under the conditions required for the purposes here in view, these compositions have either not given any deposits at all, or, particularly in the case of compositions having complex anions of chromium and mineralacids, have givenblack deposits, more or less pulvurulent, and entirely useless for industrial purposes. Only the compositions described in this present application have been found to produce the types of satisfactory deposits and the results which have been set forth in this application.

It has been found that any addition of mineral compositions to the solutions which have been here described, have the effect of deteriorating thedeposits obtained, the deterioration becoming more serious as the proportions of such additional mineral compositions added becomes larger. If to chromium plating solutions of the prior art, such as those containing chromicacid and an ammonium organic saIt there is added simply a more or less large quantity of organic acid or amine, these additions cannot change the nature of the solution, and cannot attain the results here set forth, which require a complete change of the nature of the solution. Particularly these additions do, not eliminate themineral components of the solutions of the prior art, which components are undesirable as above explained.

The chromium plating solutions which have been here described alone present the following advantages which are not to be found in thesolutions known to the prior art.

a. Deposits can be obtained with current densities as high as 300 amperes per square decimeter.

1;. Deposits can be obtained at temperatures going up to theboiling temperature of the solution.

c. Under the conditions specified in paragraphs a and b above, the solutions according to'the present invention are the only ones to give thick deposit's, extremely hard and brilliant, and this within a very short time.

d. All heretofore known plating solutions, especially of hexavalent chromium, are toxic and possess necrosive properties, which isparticularly dangerous. Inchromium plating solutions according to theipresent invention, these toxic and necrosive properties are entirely absent.

e. Since the results above enumerated under a, b, c, d, are obtained only when. the chromium is present in the state of the complex as dcscribedin the present specification, the proportions indicated in the examples as given, for the components present in the solution, are critical and of great importance since. the formation of the intended ,eomplexes depends on the employment of these specified proportions. I t

The results which have been described as having been attained with. thepresent invention, are all new results, which have not been attained With the chromium plating. solutions of the prior art, and the solutions according to the present inventiontherefore c onstitute important advan esin t ea What I claim is:

1. A chromium electroplating bath of a pH of about 7 consisting of an aqueous solution of at least one com- 9 plex salt of trivalent chromium corresponding to the formula:

Cr (Acid) (Amine) wherein acid designates at least one organic carboxylic acid radical having less than six carbon atoms, 11 designates the number of carboxylic groups in said acid radical, and amine designates at least one radical selected from the group consisting of amine and ammonium radicals, said amine radical having at most six carbon atoms.

2. A method of electrodepositing chromium upon a cathodic base which comprises passing an electrolytic current from an anode to said cathodic base through a chromium electroplating bath of a pH of about 7 consisting of an aqueous solution of at least one complex salt of trivalent chromium corresponding to the formula:

Cr (Acid) (Amine)a wherein acid designates at least one organic corboxylic acid radical having less than six carbon atoms, n designates the number of carboxylic groups in said acid radical, and amine designates at least one radical selected from the group consisting of amine and ammonium radicals, said amine radical having at most six carbon atoms. 3. A method of electrodepositing chromium upon a cathodic base which comprises passing an electrolytic current from an anode to said cathodic base through a chromium electroplating bath of a pH of about 7 consisting of an aqueous solution of at least one complex salt of trivalent chromium corresponding to the formula:

Or (Acid) 2 Amine a wherein acid designates at least one organic carboxylic acid radical having less than six carbon atoms, n designates the number of carboxylic groups in said acid radical, and amine designates at least one radical selected from the group consisting of amine and ammonium radicols, said amine radical having at most six carbon atoms, the current density on said anode and cathode exceeding one hundred and fifty amperes per square decimeter.

4. A chromium electroplating bath of a pH of about 7 consisting of an aqueous solution of a complex salt of trivalent chromium corresponding to the formula:

5. A chromium electroplating bath of a pH of about 7 consisting of an aqueous solution of a complex salt of trivalent chromium corresponding to the formula:

6. A chromium electroplating bath of a pH of about 7 consisting of an aqueous solution of a complex salt of trivalent chromium corresponding to the formula:

7. A chromium electroplating bath of a pH of about 10 7 consisting of an aqueous solution of a complex salt of trivalent chromium corresponding to the formula:

C a s 8. A chromium electroplating bath of a pH of about 7 consisting of an aqueous solution of a complex salt of trivalent chromium corresponding to the formula:

10. A method of electrodepositing chromium upon a cathodic base which comprises passing an electrolytic current from an anode to said cathodic base through a chromium electroplating bath of a pH of about 7 consisting of an aqueous solution of a complex salt of trivalent chromium corresponding to the formula:

11. A method of electrodepositing chromium upon a cathodic base which comprises passing an electrolytic current from an anode to said cathodic base through a chromium electroplating bath of a pH of about 7 consisting of an aqueous solution of a complex salt of trivalent chromium corresponding to the formula:

[Cr C204 2 (CO2CH2--CH2-CO2) (NH4 2 csHs NH -NHz) 1 12. A method of electrodepositing chromium upon a cathodic base which comprises passing an electrolytic current from an anode to said cathodic base through a chromium electroplating bath of a pH of about 7 consisting of an aqueous solution of a complex salt of trivalent chromium corresponding to the formula:

13. A method of electrodepositing chromium upon a cathodic base which comprises passing an electrolytic current from an anode to said cathodic base through a chromium electroplating bath of a pH of about 7 consisting of an aqueous solution of a complex salt of trivalent chromium corresponding to the formula:

References Cited in the file of this patent UNITED STATES PATENTS Linick Sept. 30, 1930 FOREIGN PATENTS France July 22, 1929 

1. A CHROMIUM ELECTROPLATING BATH OF A PH OF ABOUT 7 CONSISTING OF AN AQUEOUS SOLUTION OF AT LEAST ONE COMPLEX SALT OF TRIVALENT CHROMIUM CORRESPONDING TO THE FORMULA: 